Surface acoustic wave device and method of manufacture thereof

ABSTRACT

A SAW element ( 13 ) is formed of a piezoelectric substrate ( 14 ), on which are provided IDT electrodes ( 15 ), connection electrodes ( 16 ), underlying metal layers ( 17 ), and acoustic materials ( 18 ) placed on the underlying metal layers ( 17 ) and having surfaces parallel to the main surface of the piezoelectric substrate ( 14 ). The SAW element is mounted in a package ( 10 ), which is provided with external terminals ( 11 ) connected with the connection electrodes ( 16 ), and the package is hermetically sealed with a lid ( 20 ) to form a SAW device. When such a SAW element ( 13 ) is mounted faceup in a package ( 10 ) using a vacuum chuck ( 30 ), its piezoelectric substrate ( 14 ) can be protected against damage. When a SAW element ( 13 ) provided with bumps ( 23 ) on its connection electrodes ( 16 ) is mounted facedown in a package ( 10 ), the failure of electrical connections can be prevented.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a surface acoustic wave device for usein wireless communications equipment and the like and method ofmanufacture thereof.

BACKGROUND OF THE TECHNOLOGY

FIG. 23 is a cross-sectional view of a conventional surface acousticwave device (hereinafter SAW device). Referring to FIG. 23, adescription will be given below on the method of manufacture of theconventional SAW device.

First, interdigital transducer electrodes (IDT electrodes) 101 andconnection electrodes 102 are formed by forming vacuum depositedaluminum film on a disk (wafer) of piezoelectric material followed byexposing with a pattern of predetermined configuration and developing.Subsequently, acoustic absorbers 103 are formed by coating a siliconeresin on both sides of the IDT electrodes 101 by screen printing andheat treatment. In this way, a number of surface acoustic wave elements105 (SAW elements) are collectively formed on the wafer. Subsequently,the wafer is diced into individual SAW elements 105.

Next, a SAW element 105 is secured with adhesive 108 in a package 107having external terminals 106, and the external terminals 106 andconnection electrodes 102 are electrically connected with thin metalwires 109. Subsequently, opening of the package 107 is sealed with a lid110.

When the acoustic absorbers 103 are formed by screen printing in thismanner, the dimensional accuracy is poor and also their cross sectionsbecome dome-like due to drips caused by flow of the resin, thussuffering from the upper surfaces becoming curved and difficulty informing with a uniform height.

Furthermore, when mounting the SAW element 105 in the package 107, asthe SAW element 105 is transferred by sucking its surface with a vacuumchuck, there is a difficulty in sucking because the top surfaces of theacoustic absorbers 103 are curved and their heights are different. Inaddition, piezoelectric substrate 100 may incline relative to the bottomsurface of the package 107 thus presenting a possibility of causingmounting failure.

FIG. 24 is a cross-sectional view of another conventional SAW device.While thin metal wires 109 connect the connection electrodes 102 and theexternal terminals 106 in the conventional SAW device of FIG. 23, inanother conventional SAW device shown in FIG. 24, projecting electrodes111 (bumps) make the connection.

In this case, too, there is a possibility of causing connection failurewhen the heights of the acoustic absorbers 103 are non-uniform and aregreater than the bumps 111.

DISCLOSURE OF THE INVENTION

The present invention addresses the above issues and aims at providing aSAW device that can prevent mounting failure when mounting a SAW elementin a package.

In order to attain this object, the SAW device of the present inventioncomprises a package having an external terminal, a SAW element housed inthe package, and a lid for sealing opening of the package, wherein theSAW element further comprises on the surface of a piezoelectricsubstrate at least an IDT electrode, a connection electrode electricallyconnected to the IDT electrode, and an acoustic absorber formed on theoutside of the IDT electrode, that is, on an end portion of thepiezoelectric substrate, in a manner such that its top surface isparallel to the main surface of the piezoelectric substrate. As thesurface of the acoustic absorber is parallel to the main surface of thepiezoelectric substrate in this way, and as the top surface of theacoustic absorber is a plane, it is easy to suck the SAW element with avacuum chuck when mounting in a package and it is possible to securelymount it at a predetermined position of the package.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a SAW device in a first exemplaryembodiment of the present invention. FIG. 2 is a top view of the SAWdevice before sealing with a lid in the first exemplary embodiment ofthe present invention. FIG. 3 is a cross-sectional view to illustratemanufacturing process of the SAW device in the first exemplaryembodiment of the present invention.

FIG. 4 is a top view of a SAW device before sealing with a lid in asecond exemplary embodiment of the present invention. FIG. 5 is across-sectional view of the SAW device in the second exemplaryembodiment of the present invention.

FIG. 6 is a cross-sectional view of a SAW device in a third exemplaryembodiment of the present invention. FIG. 7 is a top view of the SAWdevice before sealing with a lid in the third exemplary embodiment ofthe present invention. FIG. 8 is an illustrative diagram of themanufacturing process of the SAW device in the third exemplaryembodiment of the present invention.

FIG. 9 is a top view of a SAW element in a fourth exemplary embodimentof the present invention. FIG. 10 is a cross-sectional view of a SAWdevice in the fourth exemplary embodiment of the present invention. FIG.11 is a cross-sectional view of a SAW device in the fourth exemplaryembodiment of the present invention.

FIG. 12 is a top view of a SAW element in a fifth exemplary embodimentof the present invention. FIG. 13 is a cross-sectional view of a SAWdevice in the fifth exemplary embodiment of the present invention.

FIG. 14 is a top view of a SAW element in a sixth exemplary embodimentof the present invention. FIG. 15 is a cross-sectional view of a SAWdevice in the sixth exemplary embodiment of the present invention.

FIG. 16 is a top view of a SAW element in a seventh exemplary embodimentof the present invention. FIG. 17 is a cross-sectional view of a SAWdevice in the seventh exemplary embodiment of the present invention.

FIG. 18 is a cross-sectional view of a SAW device in an eighth exemplaryembodiment of the present invention.

FIG. 19 is a top view of a SAW device before sealing with a lid in aninth exemplary embodiment of the present invention.

FIG. 20 is a top view of a SAW device before sealing with a lid in atenth exemplary embodiment of the present invention.

FIG. 21 is a top view of a SAW device before sealing with a lid in aneleventh exemplary embodiment of the present invention.

FIG. 22 is a cross-sectional view of a SAW device in a twelfth exemplaryembodiment of the present invention.

FIG. 23 is a cross-sectional view of a conventional SAW device.

FIG. 24 is a cross-sectional view of a conventional SAW device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to drawings, a description will be given below on exemplaryembodiments of the SAW device and method of manufacture thereof of thepresent invention.

First Exemplary Embodiment

Referring to FIGS. 1 to 3, a description will be given on a firstexemplary embodiment of the present invention.

A uniform thickness vapor deposited film composed of aluminum or a metalhaving aluminum as the main constituent is formed on the main surface ofa wafer composed of a piezoelectric material such as quartz, LiTaO₃,LiNbO₃, and the like. Positive type photoresist is then uniformly coatedby spin coating on top of the vapor deposited film. Next, thephotoresist is exposed and developed so as to make IDT electrodes 15having a desired shape, connection electrodes 16 to be connected to theIDT electrodes 15, and underlying metal layers 17 to be disposed at thelocations where acoustic absorber are to be formed, and then the vapordeposited film is etched to obtain IDT electrodes 15, connectionelectrodes 16, and underlying metal layers 17, and the photoresist isremoved. Here, the underlying metal layers 17 are formed to a sizegreater than the acoustic absorbers to be formed.

Subsequently, the entire main surface of the wafer whereon the IDTelectrodes 15 have been formed is covered with a negative photoresistfilm, which is then pressed while being heated. The exposed portion ofthe film resist is to become acoustic absorbers 18 and a film resisthaving the same thickness as that of the acoustic absorbers 18 are used.

Subsequently, acoustic absorbers 18 are formed by exposing anddeveloping the portion which will form the acoustic absorbers 18 in amanner such that the film resist of that portion will remain. Afterdevelopment, moisture in the acoustic absorbers 18 is removed so as toimprove adhesion with the piezoelectric substrate 14. When moisture isremaining in the acoustic absorbers 18, there is a possibility ofcausing a change in the quality of the acoustic absorbers 18 orcorrosion of the IDT electrodes 15. A plurality of SAW elements 13 areformed on the wafer in this manner.

Next, the wafer is cut into individual SAW elements 13 with a dicerwhile spraying with water. After removing moisture, a SAW element 13 istransferred by sucking its surface, that is, the surfaces of theacoustic absorbers 18 with a vacuum chuck 30 shown in FIG. 3, and ismounted in a package 10 coated with an adhesive 12. During this process,as the acoustic absorbers 18 have the same thickness and, in addition,as their top surfaces are formed parallel to the surface of thesubstrate 14, the SAW element 13 can be securely sucked with the vacuumchuck 30 and can be accurately mounted in the package 10.

Next, as illustrated in FIG. 2, the connection electrodes 16 of the SAWelement 13 and the external terminals 11 of the package 10 areelectrically connected with thin metal wires 19, the opening of thepackage 10 is sealed with a lid 20, and a SAW device shown in FIG. 1 isobtained.

Second Exemplary Embodiment

Referring to FIGS. 4 and 5, a description will be given on a secondexemplary embodiment of the present invention.

First, a plurality of SAW elements 13 having IDT electrodes 15,connection electrodes 16, and underlying metal layers 17 are formed on awafer in a manner similar to the first exemplary embodiment. Next, acoupling layer 21 is formed by uniformly spin coating a silane-basedunderlying coupling agent over the entire surface on the side of thewafer where the IDT electrodes 15 have been formed. Subsequently,solvent component in the coupling layer 21 is removed by drying.

Afterwards, as in the first exemplary embodiment, the entire mainsurface of the wafer is covered with a negative photoresist film, whichis then pressed while being heated. As the film resist, one having thesame thickness as the thickness of the acoustic absorbers 18 to beformed is used. Next, that portion of the film resist which will formthe acoustic absorber 18 is exposed and developed. After acousticabsorbers 18 have been formed, moisture in the acoustic absorbers 18 isremoved. Next, after dicing the wafer into individual SAW elements 13 asin the first exemplary embodiment, a SAW element 13 is mounted in apackage 10 and a SAW device as illustrated in FIGS. 4 and 5 is obtained.

In this exemplary embodiment, too, as in the first exemplary embodiment,the top surfaces of the acoustic absorbers 18 are formed parallel to thesurface of the piezoelectric substrate 14 thus enabling secure suctionof the SAW element 13 with the vacuum chuck 30 shown in FIG. 3 andmounting in the package 10.

Furthermore, as the silane-based coupling layer 21 formed in thisexemplary embodiment has a large force of adhesion with the acousticabsorbers 18 and the piezoelectric substrate 14, and is extremely thinas it is a monomolecular layer, it enables to greatly enhance theadhesion strength between the acoustic absorbers 18 and thepiezoelectric substrate 14 without affecting the sound absorbing effect.As a result, when dicing the wafer on which a plurality of SAW elements13 have been formed into individual SAW elements 13, peeling off theacoustic absorbers 18 from the piezoelectric substrate 14 due to sprayedwater can be prevented. In addition, acoustic absorbers 18 that arestronger to thermal stress and the like to be experienced while sealingopening of the package 10 with a lid 20 with solder, for example, ormounting a SAW device onto a circuit board by solder reflow method canbe formed.

In the meantime, although only necessary parts of the coupling layer 21are shown in FIGS. 4 and 5 for easy understanding, in this secondexemplary embodiment, the coupling layer 21 is provided over the entiresurface of the piezoelectric substrate 14 covering the IDT electrodes 15and the connection electrodes 16.

Third Exemplary Embodiment

Referring to FIGS. 6, 7, and 8, a description will be given on a thirdexemplary embodiment of the present invention.

IDT electrodes 15, connection electrodes 16, and underlying metal layers17 composed of aluminum film or aluminum alloy film are first formed ona wafer by photolithography in a manner similar to the first exemplaryembodiment. Next, as illustrated in FIG. 8, a wafer 81 on which SAWelements 13 have been formed and an electrode 85 made of stainless steeland the like are immersed in a liquid electrolyte 83, and a voltage isapplied or a current is supplied from a power supply 86 using theelectrode 85 as the cathode and a current supplying wire 87 thatcollects leads 82 form the IDT electrodes 15, connection electrodes 16and the underlying metal layers 17 as the anode. Here, the numeral 84 isa chemically resistant vessel. With this treatment, surfaces of the IDTelectrodes 15, connection electrodes 16 and the metal layers 17including respective sides are oxidized and are covered with aprotective film 22 as shown in FIG. 6. Then, the wafer 81 is pulled outfrom the liquid electrolyte 83 and cleansed by dipping in pure water.

Next, the wafer 81 is diced into individual SAW elements 13 afterforming a coupling layer 21 and acoustic absorbers 18 in a mannersimilar to the second exemplary embodiment.

Next, a SAW element 13 is mounted in a manner similar to the secondexemplary embodiment in a package 10 coated with an adhesive 12,external terminals 11 and connection electrodes 16 are electricallyconnected with thin metal wires 19 as illustrated in FIG. 7, opening ofthe package 10 is sealed with a lid 20, and a SAW device shown in FIG. 6is obtained.

In this exemplary embodiment, too, as in the first exemplary embodiment,the top surfaces of the acoustic absorbers 18 are formed parallel to themain surface of the piezoelectric substrate 14 thus enabling securesuction of the SAW element 13 with the vacuum chuck 30 shown in FIG. 3and mounting in the package 10.

Furthermore, in this exemplary embodiment, too, the silane-basedcoupling layer 21 can greatly enhance the adhesion strength between theacoustic absorbers 18 and the piezoelectric substrate 14 withoutaffecting the sound absorbing effect. As a result, peeling off theacoustic absorbers 18 from the piezoelectric substrate 14 when dicingthe wafer into individual SAW elements 13 by spraying water can beprevented. In addition, acoustic absorbers 18 that are stronger tothermal stress and the like to be experienced when solder sealing anopening of the package 10 with a lid 20, for example, or mounting a SAWdevice on a circuit board by reflow and other methods can be obtained.

In this exemplary embodiment, anodic oxidation was carried out afterforming the IDT electrodes 15 and connection electrodes 16. As thedeveloping solution for photosensitive resin is generally alkaline,there is a possibility of the IDT electrodes 15 and connectionelectrodes 16 that consist of aluminum or a metal having aluminum as themain constituent being eroded when developing the film resist that willform the acoustic absorbers 18. However, by covering the surfacesincluding the sides of the IDT electrodes 15 and the connectionelectrodes 16 with a protective film 22 consisting of alkali resistantaluminum oxide as in this exemplary embodiment, each of the electrodescan be protected against adverse effect by the developing solution.

Also, as the protective film 22 is electrically insulating,short-circuit failure of electrodes can be prevented in the event anelectrically conductive foreign object drops onto the SAW element 13.

In this exemplary embodiment, anodic oxidation was carried out after theIDT electrodes 15, connection electrodes 16 and underlying metal layers17 had been formed; however, it does not matter if anodic oxidation ofthe surface of vapor deposited film having aluminum or aluminum alloy asthe main constituent is carried out before forming the IDT electrodes15, connection electrodes 16, and underlying metal layers 17, followedby formation of the IDT electrodes 15, the connection electrodes 16, andthe underlying metal layers 17 thereby making their top surfaces coveredwith an insulating protective film. In this case, during the period ofdevelopment for forming the IDT electrodes 15, the connection electrodes16, and the underlying metal layers 17, at least surfaces of the IDTelectrodes 15 and the connection electrodes 16 will not be eroded byalkaline developing solution. However, as the sides are not covered withthe protective film 22, it is preferable to carry out anodic oxidationafter forming the IDT electrodes 15 as in the above-mentioned exemplaryembodiment in order to prevent erosion by the developing solution forforming the acoustic absorbers 18.

By carrying out anodic oxidation and covering the surface with aprotective film 22 either before or after forming the IDT electrodes 15in this way, an effect of preventing erosion of the IDT electrodes 15and the connection electrodes 16 due to alkaline developing solutionhigher than by not forming a protective film 22 can be obtained.

Fourth Exemplary Embodiment

Referring to FIGS. 9, 10, and 11, a description will be given on afourth exemplary embodiment of the present invention.

First, IDT electrodes 15, connection electrodes 16, and underlying metallayers 17 are formed on a wafer in a manner similar to the firstexemplary embodiment followed by forming acoustic absorbers 18 on theunderlying metal layers 17. The acoustic absorbers 18 have a heightenough for forming a space necessary for the IDT electrodes 15 to excitesurface acoustic waves (SAW) after being packaged in a package 10.

Next, gold bumps 23 are formed on the connection electrodes 16. Theheight of the bumps 23 is made higher than that of the acousticabsorbers 18.

Subsequently, the wafer is diced in the same way as in the firstexemplary embodiment to obtain a SAW element 13 illustrated in FIG. 9.

The SAW element 13 is then mounted with the side having the bumps 23facedown in the package 10 having external terminals 11, and theconnection electrodes 16 of the SAW element 13 and the externalterminals 11 are electrically connected through the bumps 23.

During this process, the height of the bumps 23 decreases due to heatingand pressing.

In the case of conventional acoustic absorbers 103 shown in FIG. 24, asthe height of each of the acoustic absorbers 103 differs and its crosssection is domed, in connecting connection electrodes 102 of a SAWelement 105 and external terminals 106 of a package 107 using bumps 111,there is a possibility of not being able to obtain a SAW device havingdesired characteristic since the SAW element 105 cannot be horizontallymounted in the package 107 when the height of bumps 111 has decreased toits minimum,.

However, in this exemplary embodiment, as the heights of the acousticabsorbers 18 are fixed and their top surfaces are parallel to thepiezoelectric substrate 14, in the event the height of bumps 23 haslowered as shown in FIG. 11 during the mounting process, the acousticabsorbers 18 work as stoppers and prevent the bumps 23 from becominglower than the height of the acoustic absorbers 18 thereby enabling tosecure enough space for the IDT electrodes 15 to excite SAW. Also, asthe SAW element 13 can be horizontally mounted, the dispersion ofjoining strength between the bumps 23 and the connection electrodes 16can be reduced. In other words, a SAW element 13 such as this is strongagainst mechanical and thermal stress.

Thereafter, opening of the package 10 is sealed with a lid 20 to obtaina SAW device as illustrated in FIG. 10.

While bumps 23 are formed with gold in this exemplary embodiment, theymay be formed with solder and the like.

Fifth Exemplary Embodiment

Referring to FIGS. 12 and 13, a description will be given on a fifthexemplary embodiment of the present invention.

First, IDT electrodes 15, connection electrodes 16, underlying metallayers 17, and coupling layers 21 are formed on a wafer in a mannersimilar to the second exemplary embodiment.

Next, the entire surface of the wafer is covered with a negativephotosensitive film resist and heated, and the film resist is pressed.As the film resist, one having the same thickness as that of theacoustic absorbers 18 to be formed is used. Next, the film resist isexposed and developed in a manner such that the portion of the filmresist forming the acoustic absorbers 18 will remain and the acousticabsorbers 18 are obtained. The acoustic absorbers 18 have a heightenough for the IDT electrodes 15 to form a space necessary for excitingSAW after being mounted in a package 10.

Next, the wafer is diced into individual SAW elements 13 as shown inFIG. 12 after forming bumps 23 on the connection electrodes 16 in amanner similar to the fourth exemplary embodiment, mounted in a package10, and a SAW device as shown in FIG. 13 is obtained.

In this exemplary embodiment, too, as in the fourth exemplaryembodiment, as the acoustic absorbers 18 work as stoppers at the timethe SAW element 13 is being mounted, the height of the bumps 23 will notbecome lower than the height of the acoustic absorbers 18 and a spacenecessary for the IDT electrodes 15 to excite SAW can be secured. Also,during this process, as the SAW element 13 can be horizontally mounted,the dispersion of adhesion strength between the bumps 23 and theconnection electrodes 16 can be reduced. In other words, a SAW devicesuch as this is strong against mechanical and thermal stress.

Furthermore, in this exemplary embodiment, as in the second exemplaryembodiment, by forming a coupling layer 21 between the underlying metallayers 17 and the acoustic absorbers 18, the adhesion strength betweenthe acoustic absorbers 18 and the piezoelectric substrate 14 can begreatly enhanced without affecting the sound absorbing effect. As aresult, peeling off the acoustic absorbers 18 from the piezoelectricsubstrate 14 by spraying water when dicing the wafer into individual SAWelements 13 can be prevented. In addition, acoustic absorbers 18 thatare stronger to stress such as thermal stress to be experienced whilesolder sealing opening of the package 10 with a lid 20, for example, ormounting a SAW device on a circuit board by reflow and other methods canbe provided.

Although only necessary parts of the coupling layer 21 are shown inFIGS. 12 and 13 for ease of understanding, in this exemplary embodimentthe coupling layer 21 is provided over the entire surface of thepiezoelectric substrate 14 covering the IDT electrodes 15 and theconnection electrodes 16.

Sixth Exemplary Embodiment

Referring to FIGS. 14 and 15, a description will be given on a sixthexemplary embodiment of the present invention.

First, IDT electrodes 15, connection electrodes 16, and underlying metallayers 17 of which the surfaces and the sides have been covered with aninsulating protective film 22 are formed in a manner similar to thethird exemplary embodiment.

Going through the same subsequent processes as in the fourth and fifthexemplary embodiments, acoustic absorbers 18 are formed on theunderlying metal layers 17. After further forming bumps 23 on theconnection electrodes 16, wafer is diced into individual SAW elements 13shown in FIG. 14. Next, a SAW device as shown in FIG. 15 is fabricatedby mounting a SAW element 13 in a package 10 and sealing with a lid 20.

In this exemplary embodiment, as in the third exemplary embodiment,since the surfaces and sides of the IDT electrodes 15, connectionelectrodes 16, and underlying metal layers 17 are covered with aprotective film 22, erosion of the IDT electrodes 15 and the connectionelectrodes 16 by alkaline liquid electrolyte during development for theformation of the acoustic absorbers 18 can be prevented.

Also, as described in the third exemplary embodiment, the electricallyinsulating protective film 22 may be formed on the surfaces of the IDTelectrodes 15 and the connection electrodes 16 by carrying out anodicoxidation after a metal film of aluminum or aluminum alloy has beenformed on the wafer prior to the formation of the IDT electrodes 15,connection electrodes 16, and underlying metal layers 17.

Seventh Exemplary Embodiment

Referring to FIGS. 16 and 17, a description will be given on a seventhexemplary embodiment of the present invention. In this exemplaryembodiment, only the difference from the SAW device of the fourthexemplary embodiment will be described.

In this exemplary embodiment, as shown in FIG. 16, underlying metallayers 17 and acoustic absorber 18 to be formed thereon are formed inthe shape of a frame on the periphery of a piezoelectric substrate 14 ina manner such that they surround IDT electrodes 15 and connectionelectrodes 16. The method of formation is the same as in the fourthexemplary embodiment.

After a SAW element 13 has been mounted in a package 10, a filler 25composed of a silicone-based resin is filled between the SAW element 13and the inner wall of the package 10 and then cured by heating. Duringthis process, the frame-shaped acoustic absorber 18 prevents the filler25 from flowing into the SAW excitation space of the IDT electrodes 15.

In addition, the filler 25 has, after being cured, a higher elasticitythan the acoustic absorber 18 and can absorb spurious waves that havenot been absorbed by the acoustic absorber 18 and can also relieve thestress applied to the SAW element 13 due to the difference in thermalexpansion coefficients between those of the SAW element 13 and thepackage 10 thus preventing change of characteristics.

Eighth Exemplary Embodiment

Referring to FIG. 18, a description will be given on an eighth exemplaryembodiment of the present invention.

The difference of a SAW device of this exemplary embodiment from the SAWdevice of the seventh exemplary embodiment lies in that a filler 25 isprovided not only between the SAW element 13 and the inner wall of thepackage 10 but also on the rear surface of the SAW element 13 facing alid 20. Other structure is the same. As a result, a further highereffect of absorbing spurious waves is obtained compared with that of theSAW device of the seventh exemplary embodiment.

While the filler 25 is provided in this exemplary embodiment over theentire rear surface of the SAW element 13 facing the lid 20, similareffect of absorbing spurious waves is obtainable by providing only onthe portions corresponding to the IDT electrodes 15.

Also, when sealing the package 10 with the lid 20, it is preferable thatcare be taken not to make the filler 25 provided on the rear surface ofthe SAW element 13 facing the lid 20 and the lid 20 come in contact witheach other in order to prevent deterioration of the characteristicscaused by deformation of the SAW element 13 due to an excessive pressureapplied from the rear surface of the SAW element 13.

Ninth Exemplary Embodiment

Referring to FIG. 19, a description will be given on a ninth exemplaryembodiment of the present invention.

In this exemplary embodiment, as illustrated in FIG. 19, two pairs ofIDT electrodes 15 are integrated into one unit, two surface acousticwave units (SAW units) 27 provided with reflector electrodes 26 on bothsides of them are provided in parallel on a piezoelectric substrate 14,and an acoustic absorber 18 is formed on the piezoelectric substrate 14between the SAW units 27 with intervention of an underlying metal layer17. By employing this structure, acoustic coupling between the two SAWunits 27 can be suppressed, providing a superior quantity of out-of-bandattenuation.

As the acoustic absorber 18 is made by the method described in the firstexemplary embodiment and the top surface is parallel to the surface ofthe piezoelectric substrate 14, it enables mounting of a SAW element 13in a package 10 by securely sucking the SAW element 13 with a vacuumchuck 30 shown in FIG. 3.

In a SAW device having a plurality of SAW units 27 on a singlepiezoelectric substrate 14, it is preferable to provide an acousticabsorber 18 at least between opposing IDT electrodes 15 between the twoSAW units 27 and suppress acoustic coupling. Also, in the SAW units 27,when reflector electrodes 26 are provided between the IDT electrodes 15or at both ends of the IDT electrodes 15, it is preferable to provide anacoustic absorber 18 fabricated by a similar method not only between theIDT electrodes 15 between the SAW units 27 as described above but alsobetween the reflector electrodes 26.

Tenth Exemplary Embodiment

Referring to FIG. 20, a description will be given on a tenth exemplaryembodiment of the present invention.

In this exemplary embodiment, acoustic absorbers 18 are provided notonly between two SAW units 27 but also between reflector electrodes 26and the end of a piezoelectric substrate 14 in the direction ofpropagation of SAW as illustrated in FIG. 20. Needless to say, theacoustic absorbers 18 are provided on the piezoelectric substrate 14with intervention of underlying metal layers 17 in a manner similar tothe first exemplary embodiment. As these acoustic absorbers 18 and theacoustic absorber 18 between the SAW units 27 are of the same thicknessand their main surfaces are formed parallel to the surface of thepiezoelectric substrate 14, it enables secure sucking of a SAW element13 with a vacuum chuck shown in FIG. 3 and mounting in a package 10.

By employing this structure, the effect of absorbing spurious waves canbe further enhanced compared with the SAW device of the ninth exemplaryembodiment.

Eleventh Exemplary Embodiment

Referring to FIG. 21, a description will be given on an eleventhexemplary embodiment of the present invention.

When trying to minimize the inductance between connection electrodes 16and external terminals 11 by shortening thin metal wires 19 connectingthe connection electrodes 16 and the external terminals 11, a wiringelectrode is generally provided from the connection electrodes 16 to anend of a piezoelectric substrate 14 which is close to the externalterminals 11 to be connected.

However, by connecting the connecting electrodes 16 with underlyingmetal layers 17, and connecting the external terminals 11 to beconnected with the underlying metal layers 17 using thin metal wires 19as in this exemplary embodiment illustrated in FIG. 21, the thin metalwires 19 can be shortened without newly providing a wiring electrode.

In this way, downsizing of a SAW device can be achieved.

Here, the SAW device of this exemplary embodiment is fabricatedaccording to the method of manufacture described in the first exemplaryembodiment with the exception that the connection electrodes 16 and theunderlying metal layers 17 are made in a connected state. As acousticabsorbers 18 provided between IDT electrodes 15 and an end of apiezoelectric substrate 14 are of the same thickness as in the firstexemplary embodiment, and the top surfaces of them are formed parallelto the surface of the piezoelectric substrate 14, it enables securesucking of a SAW element 13 with a vacuum chuck shown in FIG. 3 andmounting of it in a package 10.

Twelfth Exemplary Embodiment

Referring to FIG. 22, a description will be given on a twelfth exemplaryembodiment of the present invention.

In this exemplary embodiment, an antireflective film 28 of amorphoussilicon or silicon nitride that absorbs light well is formed over theentire rear surface of a wafer composed of crystal, LiTaO3, LiNbO3, orthe like.

A uniform thickness vapor deposited film consisting of aluminum or ametal having aluminum as the main constituent is formed on the mainsurface of such a wafer. Subsequently, a positive photoresist isuniformly spin coated on the vapor deposited film.

Next, the photoresist is exposed in a manner such that IDT electrodes 15having a desired shape and connection electrodes 16 to be connected tothe IDT electrodes 15 can be formed. In this case, as the antireflectivefilm 28 has been formed on the rear surface of the wafer, it absorbs thelight passing the wafer and prevents light from reflecting on the mainsurface of the wafer.

Next, the photoresist is developed to form IDT electrodes 15 andconnection electrodes 16, and an electrically insulating protective film22 is formed by anodic oxidation on the surface of the IDT electrodesand connection electrodes 16 including their sides.

Subsequently, in the same manner as in the second exemplary embodiment,coupling layers 21 are formed at least at the location on the wafersurface where acoustic absorbers 18 are to be formed.

Next, the entire surface of the wafer on which the IDT electrodes 15have been formed is covered with a negative photosensitive film resist,which is then pressed while being heated. As the film resist, one havingthe same thickness as that of the acoustic absorbers 18 to be formed isused.

Next, the portion of the film resist that will form the acousticabsorbers 18 is exposed. In this case, too, the antireflective film 28on the rear surface of the wafer absorbs light passing through the waferand prevents it from reflecting on the main surface of the wafer.

Afterwards, development is made to obtain acoustic absorbers 18. Afterdevelopment, moisture in the acoustic absorbers 18 is removed to improveadhesion with a piezoelectric substrate 14.

Thereafter, a SAW device shown in FIG. 22 is fabricated in the samemanner as in the first exemplary embodiment.

In this SAW device, as the acoustic absorbers 18 on both sides of theIDT electrodes 15 have the same thickness as in the first exemplaryembodiment and are formed in a manner such that their top surfaces areparallel to the surface of the piezoelectric substrate 14, it enablessecure sucking of a SAW element 13 with a vacuum chuck 30 shown in FIG.3 and mounting of it in a package 10.

In the meantime, although the antireflective film 28 is formed in thisexemplary embodiment before a vapor deposited metal film is formed,acoustic absorbers 18 having superior configurational accuracy can beobtained by forming the antireflective film 28 before exposing the filmresist that will form the acoustic absorbers 18. However, the IDTelectrodes 15 and the connection electrodes 16 can be formed with higheraccuracy when the antireflective film 28 is formed before forming theIDT electrodes 15.

Also, although the antireflective film 28 is formed over the entire rearsurface of the wafer, it is preferable to form it larger than at leastthe portions that will form the acoustic absorbers 18, IDT electrodes15, and connection electrodes 16 so that transmitting light thatreflects on the rear surface of the wafer will not impinge on thephotosensitive resin that will form the acoustic absorbers 18 and theIDT electrodes 15 and the photosensitive resin that will form theconnection electrodes 16.

A description of the gist of the present invention will be given in thefollowing.

(1) In each of the above-mentioned exemplary embodiments, the acousticabsorbers 18 are formed using a film type negative photosensitive resinin order that they can be stably fabricated with a higher soundabsorbing effect, smaller area, and lower profile. However, similareffect is obtainable by coating on a wafer a photosensitive resinsolution by spin coating and the like to a uniform thickness. At thistime, as the thickness achievable by a single spin coating is small,spin coating of the photosensitive resin solution may be repeatedseveral times to adjust to a desired thickness of the acoustic absorbers18.

(2) In each of the above-mentioned exemplary embodiments, the acousticabsorbers 18 are formed using a photosensitive resin selected from thegroup consisting of epoxy resin, acrylic resin, and polyimide resin thatare resilient and can efficiently absorb spurious waves. In particular,acrylic resin is preferable because of its superior adhesion to apiezoelectric substrate 14.

(3) While the acoustic absorbers 18 may be formed in a nearly squareshape, it is preferable to form them in a manner such that the endportions of the acoustic absorbers 18 on the side of the IDT electrodes15 are saw-toothed as shown in each of the above exemplary embodimentsso that scattering effect can be obtained in addition to sound absorbingeffect, thereby enhancing suppression of spurious waves. Also, peelingoff the acoustic absorbers 18 from the piezoelectric substrate 14 bypressure of water spray, for instance, applied when dicing a wafer canbe prevented by making the shape as cornerless as possible, and making acorner obtuse even when forming a corner as described in each of theabove exemplary embodiments.

(4) A sufficient sound absorbing effect can be obtained by making thesmallest width portion of the acoustic absorbers 18 in the direction ofSAW transmission equal to or greater than 0.5λ(λ=SAW wavelength).

(5) It is preferable to make the length of the acoustic absorbers 18 inthe direction orthogonal to the direction of SAW transmission equal toor greater than the length of the IDT electrodes 15 in the samedirection. This is because, as SAW has a diffraction effect as aproperty of waves, a sufficient sound absorbing effect is obtained bymaking the length equal to or greater than the length of the IDTelectrodes 15 in the same direction.

(6) In forming the acoustic absorbers 18 having superior configurationalaccuracy by photolithographic method as in the present invention, atleast one of the following three methods can be employed.

The first is a method in which an underlying metal layer 17 is formed onthe surface of a wafer in order not to allow light for film resistexposure from being transmitted to the rear surface of the wafer duringthe process of forming the acoustic absorbers 18. This method is mostpreferable as the underlying metal layer 17 can be formed simultaneouslywith the formation of the IDT electrodes 15 and connection electrodes 16thus making a separate process unnecessary.

The second is a method in which rear surface of the wafer is roughenedto scatter reflected light so as to avoid impinging of reflected lightfrom the rear surface of the wafer when exposing film resist for makingacoustic absorbers.

The third is a method in which an antireflective film 28 is formed onthe rear surface of the wafer so as to absorb transmitted light.

When forming the underlying metal layer 17 and the antireflective film28, the size of the antireflective film 28 is made larger than theacoustic absorbers 18 to be formed in order to ensure the above effect.

(7) In forming the acoustic absorbers 18, when using a film resist whichis equal to or smaller in size than the wafer but larger than theportion making SAW element 13, or when using a film resist larger thanthe wafer, it is preferable to cut it after putting it to the wafer to asize equal to or smaller than the wafer and larger than the portionforming a SAW element 13. The reason is because, if the film resist islarger than the wafer, there is a possibility that the film resist maypeel off, or smooth transfer may be hindered, by catching on thetransfer device when transferring the wafer for exposure anddevelopment.

(8) In the second or the fifth exemplary embodiment, a coupling agent iscoated over the entire surface of the piezoelectric substrate 14. As aresult, a coupling layer 21 is formed not only between the piezoelectricsubstrate 14 and the acoustic absorbers 18 but also on the surfaces ofthe IDT electrodes 15. However, as the coupling layer 21 does not do anyharm on the characteristics of the SAW device, it is not necessary toremove it.

(9) It is preferable to make the bottom surfaces (on the side of thepiezoelectric substrate 14) larger than the top surfaces of the acousticabsorbers 18 thereby to improve adhesion strength with the piezoelectricsubstrate 14.

(10) Although a silicone resin is used as the filler 25, otherthermosetting resin may also be used. However, it is preferable that thefiller 25 is a material that has higher resilience than that of theacoustic absorbers 18 so that stress applied to the SAW element 13 dueto thermal expansion and the like can be relieved.

(11) When heating or cooling the piezoelectric substrate 14, it ispreferable to avoid abrupt temperature change in order to preventpyroelectric destruction.

(12) By carrying out anodic oxidation on the surfaces of the IDTelectrodes 15, connection electrodes 16, and underlying metal layers 17and covering their surfaces with an electrically insulating layer,short-circuit between electrodes can be prevented in the event anelectrically conductive foreign object drops on the surface of the SAWelement 13.

INDUSTRIAL APPLICABILITY

According to the present invention, by forming acoustic absorbers havingthe same thickness and main surfaces parallel to the surface of apiezoelectric substrate, mounting failure can be prevented when mountinga SAW element in a package.

LIST OF REFERENCE NUMERALS

10. Package

11. External terminal

12. Adhesive

13. SAW element (surface acoustic wave element)

14. Piezoelectric substrate

15. IDT electrode (interdigital transducer electrode)

16. Connection electrode

17. Underlying metal layer

18. Acoustic absorber

19. Thin metal wire

20. Lid

21. Coupling layer

22. Protective film

23. Bump (projecting electrode)

25. Filler

26. Reflective electrode

27. Surface acoustic wave unit

28. Antireflective film

30. Vacuum chuck

81. Wafer

82. Lead

83. Liquid electrolyte

84. Chemically resistant vessel

85. Metal electrode

86. Power supply

87. Current supply wire

100. Piezoelectric substrate

101. IDT electrode (interdigital transducer electrode)

102. Connection electrode

103. Acoustic absorber

105. SAW element (surface acoustic wave element)

106. External terminal

107. Package

108. Adhesive

109. Thin metal wire

110. Lid

111. Bump (projecting electrode)

What is claimed is:
 1. A surface acoustic wave device comprising: apackage having an external electrode; a surface acoustic wave elementhoused in said package; and a lid sealing opening of said package; saidsurface acoustic wave element further comprising on the surface of apiezoelectric substrate: at least an interdigital transducer electrode;a connection electrode electrically connected to said interdigitaltransducer electrode; and acoustic absorbers formed on both sides ofsaid interdigital transducer electrode, wherein said interdigitalelectrode has protective surface on its surface, said acoustic absorbersare formed with a photosensitive resin, the top surfaces of saidacoustic absorbers are parallel to the main surface of saidpiezoelectric substrate, and said connection electrode is electricallyconnected to said external electrode.
 2. The surface acoustic wavedevice of claim 1, wherein said protective layer has a higher resistanceto developing solution for the photosensitive resin constituting saidacoustic absorbers than that of the main metal constituting theinterdigital transducer electrode.
 3. The surface acoustic wave deviceof claim 2, wherein said photosensitive resin is negative type.
 4. Thesurface acoustic wave device of claim 2, wherein said photosensitiveresin is one selected from the group consisting of epoxy resin,polyimide resin, and acrylic resin.
 5. The surface acoustic wave deviceof claim 1, wherein the width of said acoustic absorbers in thedirection of the transmission of surface acoustic waves is0.5λ(λ=wavelength of surface acoustic waves) or greater.
 6. The surfaceacoustic wave device of claim 1, wherein the end portion of saidacoustic absorbers on the side of the interdigital transducer electrodeis saw-toothed.
 7. The surface acoustic wave device of claim 1, whereina coupling layer having adhesion strength greater than the adhesionstrength between said acoustic absorbers and said piezoelectricsubstrate is disposed at least between said acoustic absorbers and saidpiezoelectric substrate excluding the bottom portion of saidinterdigital transducer electrode.
 8. The surface acoustic wave deviceof claim 7, wherein said coupling layer has a surface area larger thanthat of said acoustic absorbers.
 9. The surface acoustic wave device ofclaim 7, wherein said coupling layer is formed using a silane-basedresin.
 10. The surface acoustic wave device of claim 1, wherein anunderlying metal layer is provided between said acoustic absorbers andsaid piezoelectric substrate.
 11. The surface acoustic wave device ofclaim 10, wherein said interdigital transducer electrode and saidunderlying metal layer are connected, and said underlying metal layerand said external terminal are connected.
 12. The surface acoustic wavedevice of claim 1, wherein an antireflective film is provided on therear surface of said piezoelectric substrate opposite said acousticabsorbers with intervention of said piezoelectric substrate.
 13. Thesurface acoustic wave device of claim 12, wherein the area of formingsaid antireflective film is made larger than said acoustic absorbers.14. The surface acoustic wave device of claim 13, wherein saidantireflective film is formed with amorphous silicon or metal nitridefilm.
 15. The surface acoustic wave device of claim 1, wherein the areaof the bottom surface of said acoustic absorbers that come in contactwith said piezoelectric substrate is made larger than that of the topsurface.
 16. The surface acoustic wave device of claim 1, whereinelectrical connection between said connection electrode and saidexternal terminal of the package is made using a bump formed on saidconnection electrode of said surface acoustic wave element.
 17. Thesurface acoustic wave device of claim 1, wherein said acoustic absorbersare provided on the outer periphery of said piezoelectric substrate in amanner such that they enclose said interdigital transducer electrode andsaid connection electrode.
 18. The surface acoustic wave device of claim17, wherein a filler is provided between the inner wall of said packageand the side of said piezoelectric substrate.
 19. The surface acousticwave device of claim 18, wherein said filler has resilience higher thanthat of said acoustic absorbers.
 20. The surface acoustic wave device ofclaim 19, wherein silicone resin is used as said filler.
 21. The surfaceacoustic wave device of claim 16, wherein acoustic absorbrs are providedat least on a region on the rear surface of said piezoelectric substrateopposite said interdigital transducer electrode.
 22. The surfaceacoustic wave device of claim 21, wherein said acoustic absorbersprovided on the rear surface of said piezoelectric substrate and a lidare not in contact.